1. Field
This document relates to an apparatus for controlling the constant current for multi-channel LEDs (Light Emitting Diodes) and a liquid crystal display using the same.
2. Related Art
Since an LED is a current driving element, it is driven by constant current to keep the luminance constant. A plurality of LEDs may be configured in multiple channels and implemented as light sources. Each LED channel comprises an LED array having a plurality of LEDs connected. The LED channels may be commonly connected to a DC-DC converter.
There may be a difference in LED driving voltage, i.e., forward voltage Vf, between the LED channels due to a difference between the LEDs. The DC-DC converter generates an output voltage with respect to the LED array of an LED channel with a higher forward voltage Vf. To keep the current flowing through the LED channels constant, a transistor connected to the LED array for each channel is controlled to control the constant current flowing through the LED channels. To this end, a voltage equal to or higher than a predetermined reference voltage is set for the base of a transistor provided in each LED channel to control the current flowing through the LED channels.
In a conventional apparatus for controlling constant current for multi-channel LEDs, a voltage higher than a required level may be applied to two ends of a transistor for driving constant current, due to a forward voltage difference between the LED channels. In this case, the transistor needs to be larger in size because it requires high-capacity channel. As a result, transistors of the conventional apparatus for controlling the constant current for multi-channel LEDs are not integrated within an integrated circuit (hereinafter, “IC”), but instead disposed outside the IC.
FIG. 1 is a view showing an example of power consumption for each LED channel in a conventional apparatus for controlling the constant current for multi-channel LEDs.
In the apparatus for controlling the constant current for multi-channel LEDs, as shown in FIG. 1, it is assumed that the forward voltage Vf1 of an LED array LED1 of a first LED channel is 9 V, the forward voltage Vf2 of an LED array LED2 of a second LED channel is 8 V, and the current I flowing through the LED arrays LED1 and LED2 when transistors Q1 and Q2 are turned on is 0.1 A. In this case, a DC-DC converter (not shown) outputs a voltage of 10 V required to drive the first and second LED channels, with respect to the LED array of the first LED channel with higher forward voltage Vf1.
Electric power PLED1 consumed in the LED array LED1 of the first LED channel is “PLED1=9 V×0.1 A=0.9 W”. Here, 0.1 W is the minimum electric power which is lost when only a collector-emitter voltage Vce1 enabling current control in a linear operating region of the transistor Q1 is used. Since the voltage Vr1 of an emitter resistor R1 is fixed to “Vr1=5Ω×0.1 A=0.5 V”, the power Pr1 consumed in the emitter resistor R1 is “Pr1=0.5 V×0.1 A=0.05 W”. The collector-emitter voltage Vce1 of the transistor Q1 is equal to a voltage of 0.5 V, which is obtained by subtracting the forward voltage Vf1 of the LED array LED1 and the voltage Vr1 of the emitter resistor R1 from the output voltage 10V of the DC-DC converter. Accordingly, since Vec1=0.5 V, the power consumption Pq1 of the transistor Q1 is equal to Pq1=Vce1×I=0.5 A×0.1 A=0.05 W.
As electric power PLED2 consumed in the LED array LED2 of the second LED channel is Vf2=8 V, “PLED2=8V×0.1 A=0.8 W”. Since he voltage vr2 of an emitter resistor R2 is fixed to Vr2=5Ω×0.1 A=0.5 V”, the power Pr2 consumed in the emitter resistor R2 is “Pr2=0.5 V×0.1 A=0.05 W”. If the reference voltage of the second LED channel is increased to 1.5 V, the collector-emitter voltage Vce2 of the transistor Q2 is equal to a voltage of 1.5 V, which is obtained by subtracting the forward voltage Vf2 of the LED array LED2 and the voltage Vr2 of the emitter resistor R2 from the output voltage 10V of the DC-DC converter. Accordingly, since Vec2=1.5 V, the power consumption Pq2 of the transistor Q2 is equal to Pq2=Vce2×I=1.5 A×0.1 A=0.15 W.
As seen from FIG. 1, the transistor Q2 of the second LED channel has higher power consumption than that Q1 of the first LED channel due to the forward voltage difference between the LED arrays LED1 and LED2, and the amount of heat generation is as much increased. Accordingly, the transistors Q1 and Q2 become larger in size because they need to be manufactured to have such a channel ratio as to withstand as much power consumption as the rated power consumption of the second transistor Q2, and this makes it difficult to integrate the transistors within an IC.